1. Field of the Invention
This invention is related to a control system for an automobile automatic transmission, and, in particular, related to an automatic transmission control system for coupling a frictional coupling element of an automatic transmission to provide different torque transmissive capacities in different gear stages.
2. Description of Related Art
In automatic transmissions for automobiles, a transmission gear mechanism is coupled to a torque converter, and a drive torque transmission path of the transmission gear mechanism is changeable through the selective coupling of a plurality of frictional coupling elements so as to automatically shift the automatic transmission to various gears according to vehicle driving conditions. In such an automatic transmission, a hydraulic controlling circuit controls coupling pressure supplied to the frictional coupling elements. Japanese Unexamined Patent Publication No. 1-150055 reveals an hydraulic control circuit of this type, which has a linear solenoid valve to control coupling pressure supplied to each of the frictional coupling elements. This hydraulic control circuit includes a regulating valve disposed in a pressure lines to the frictional coupling elements. The control pressure supplied to the regulating valve is developed by means of the linear solenoid valve. In this instance, a control signal is output to the linear solenoid valve from a controller so as to develop a control pressure of the regulating valve, thereby appropriately regulating the coupling pressure applied to the frictional coupling elements. This enables the automatic transmission to smoothly couple and uncouple the frictional coupling elements, so as to reduce shift shock. In addition, through the regulation of torque transmissive capacity of the frictional coupling element relative to an input torque to the frictional coupling element, the frictional coupling element reliably transmits torque without loss in driving force of an oil pump attributable to an unnecessary increase in torque transmissive capacity.
Automatic transmissions of this kind, some of these frictional coupling elements, for instance a low reverse brake, which is coupled for a first gear for the application of engine brake and a reverse gear, vary its torque transmissive capacity between the different gears, the first gear and the reverse gear. Since the low reverse brake, to which the engine output torque is input, has a high reduction ratio, it is generally necessary to have a large transmissive capacity of torque. However, since only an inertial torque is input during the application of engine brake, the low reverse brake may have sufficient to have a relatively low torque transmissive capacity.
Consequently, in order to regulate appropriately the coupling pressure to the low reverse brake by the aid of the regulating valve and the linear solenoid valve described above, the range of pressure variation between a pressure necessary to develop the maximum torque transmissive capacity for the reverse gear and a pressure necessary to develop the minimum torque transmissive capacity for the first gear is very wide. In addition, in the event that the coupling action of the low reverse brake effects to shift shock when shifting from a neutral gear to a reverse gear, in order for the transmission to reduce the shift shock, the coupling pressure to the low reverse brake must be held at a relatively low level for a while during shifting to the reverse gear. In this instance, the linear solenoid valve must cover a more widened range of pressure variation. Such a wide range of pressure variation disables the linear solenoid valve to control the coupling pressure with high accuracy.